Experimenting with external interrupts on a Mega. Pins 2 and 3 work with standard


coding and call my ISR. With pins 18 and 19 I had to add

EIFR = 4;

for pin 19, and

EIFR = 8;

for pin 18 before calling attachInterrupt(...) to get them to work. Why?

BTW, I'll add this table that I filled in by trial and error for anyone interested in Atmel (AVR) external interrupt numbers to Arduino external interrupt numbers (I won't list the AVR chip pin numbers)

AVR INT      5 .  4 .  3 .  2 .  1 .  0
EIFR (Dec)  32 . 16 .  8 .  4 .  2 .  1
Arduino Pin  3 .  2 . 18 . 19 . 20 . 21
Arduino INT  1 .  0 .  5 .  4 .  3 .  2

I know writing a 1 to the EIFR bit for the appropriate interrupt clears the interrupt. When I reset the Arduino without clearing interrupts on pins 2 and 3 it's fine. If I don't clear 18 and 19 after a reset, it throws the interrupt right away. (Yes, I should OR the EIFR with the bit I want to change but I'm just playing) Didn't try 20 and 21 because I'm using them for comm.


It's not that pins 18 or 19 are faulty, you are just lucky the other ones worked.

As I mention on my page about interrupts* you are recommended to always write 1 to the appropriate EIFR bit before enabling interrupts because it (the interrupt condition) may have been set in the past.

I checked the source for attachInterrupt in the file WInterrupts.c and there is no mention of the EIFR register, therefore attachInterrupt never affects that register.

* Also see my answer: How do interrupts work on the Arduino Uno and similar boards?

  • Wow! I know where I'll be spending the next few hours. Thanks! – 4redwings Jan 8 at 13:17

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